I. Field of the Invention
This invention relates to a streak camera unit with a streak tube which, for instance, is suitable for measuring a weak light beam which changes repeatedly with the same period and in the same pattern.
II. Background Information
A streak camera has been known as a device for measuring the temporal variation in intensity of a light emission which changes at high speed.
The streak camera includes an electron tube which is called a streak tube. The streak tube has a photocathode at one end, a phosphor screen (layer) at the other end and a pair of deflection electrodes are disposed therebetween.
When a light beam is applied to the photocathode of the streak tube, the photocathode emits photoelectrons as a function of the incident light beam. Thus the photoelectron beam changes in proportion to the intensity of the incident light beam.
When the photoelectron beam is passed through the electric field formed by the deflection electrodes while advancing towards the phosphor screen, it is deflected in one direction, resulting in the sweep on the phosphor screen. As a result the change in intensity of the incident light beam appears as the change in luminance of the phosphor screen in the direction of sweep (i.e., the direction of the time axis). This is a so-called "streak image." The streak image is photographed with a camera or detected with a TV (television) camera, so that the distribution of brightness or luminance of the streak image in the direction of sweep can be quantized for measurement of the change in intensity of the light beam.
The above-described streak tube is utilized in a so-called "synchroscan streak camera." The synchroscan streak camera is used to measure a weak light beam which is periodically produced. An example of the weak light beam of this type is fluorescence provided through high repetition laser pulse excitation. When a light beam under test is low in intensity, its streak image is also weak, and therefore it is difficult to accurately obtain its intensity distribution.
When the light beam to be measured is a pulsed light beam which occures with the same waveform and with the same period, the sine wave voltage whose period is coincident with that of the pulsed light beam and whose phase is in constant relation with that of the pulsed light beam is applied to the deflection electrodes of the streak tube. In this case, the streak images, having the same intensity distribution in the direction of sweep(i.e., the direction of time axis), can be superimposed at the position on the output phosphor screen. If the streak images are integrated n times, the streak image brightness(or optical energy) on the output screen is substantially increased by a factor of n, and therefore even a considerably weak light emission can be observed with a satisfactory signal to noise (SIN) ratio.
The high repetition laser employed usually is a mode locked dye laser having a repetition frequency of about 100 MHz. In this case, for instance in a one-second measurement, the integration can be made 100,000,000 times. The synchroscan streak camera is based on the above-described principle.
FIG. 8 is a block diagram of a synchroscan streak camera with its streak tube sectioned along the plane which includes the optical axis.
As shown in FIG. 8, a cylindrical housing 81 has a photocathode 82 formed on the inner surface of its other end which is transparent. A voltage which is lower than the ground potential is applied to the photocathode 82 from a power source E.sub.2.
A mesh electrode 83 is disposed adjacent to the photocathode 82. In order to accelerate photo-electrons emitted from the photocathode 82, a voltage higher than that of the photocathode 82 is applied to the mesh electrode 83 from a power source E.sub.1. A focus electrode 84 is arranged between the mesh electrode 83 and an anode plate 85 having an opening at the center. The anode plate 85 is grounded. Some part of the voltage of source E.sub.2 is applied to the focus electrode 84 so that the focus electrode 84 serves as an electron lens which focuses the photoelectrons emitted from the photocathode 82 on the phosphor screen 87.
A pair of deflection electrodes 86a and 86b made up of a pair of flat plates are disposed adjacent to the anode plate 85. A periodically varying voltage is applied across the deflection electrodes by a deflecting voltage generating means 88.
FIGS. 9A, 9B and 9C show a graphical representation to assist in explaining the operation of the synchroscan streak camera which is described above. In an ordinary synchroscan streak camera, the deflecting voltage generating means 88 produces a sine wave voltage as indicated in FIG. 9B. The parts p.sub.1 -q.sub.1, p.sub.2 -q.sub.2 . . . and p.sub.n -q.sub.n of the sine wave voltage which change from positive to negative are used to deflect the electron beam from the upper edge to the lower edge of the phosphor screen 87.
The deflecting voltage is selected so that its frequency is the same as the repetitive frequency of a light beam to be measured, and its phase is in synchronism with the period of the beam.
In order to observe the light emission phenomenon shown in FIG. 9A, a sine wave voltage as shown in FIG. 9B is applied across the deflection electrodes 86a and 86b. This sine wave voltage which has a repetitive period can be generated synchronously in phase with a laser beam for exciting an object to be observed for instance, FIG. 9C shows the luminance distributions in the direction of the time axis on the phosphor screen 87 which are produced when the screen 87 is swept with the electron beam.
Assuming the optical intensity of the object under observation is low, the changes in the luminance distribution on the phosphor screen 87 which is provided at the first sweep with the part p.sub.1 -q.sub.1 will be quite small as shown on screen (1) of FIG. 9C and often will not be detectable with the naked eye.
As the above-described operation is repeated, the luminance distribution becomes clear as is apparent from screens (2) and (3) of FIG. 9C. Theoretically, when the sweep is repeated n times, the luminance is approximately n times as great as that provided on the first sweep.
If the light beam under measurement is emitted for the sweep return periods s.sub.1 -t.sub.1, s.sub.2 -t.sub.2, . . . and s.sub.n -t.sub.n of the sine wave sweep voltage synchronous with the period T, shown in FIG. 9B, the streak image formed by the parts s.sub.1 -t.sub.1, s.sub.2 -t.sub.2, . . . and s.sub.n -t.sub.n will lie on that formed by the parts p.sub.1 -q.sub.1, p.sub.2 -q.sub.2, . . . p.sub.n -q.sub.n. However, these streak images are reversed in the time axis direction on the phosphor screen. Therefore, in this case, the images do not add and the measurement cannot be accomplished.
The above-described difficulty can be eliminated by employing a circularscan system such as is shown in FIG. 10. In FIG. 10, parts corresponding functionally to those which have been already described with reference to FIG. 8 are designated by corresponding reference numerals or characters.
The streak tube has, in addition to the above-described streak deflection electrodes 86a and 86b, another pair of deflection electrodes 89a and 89b which deflect the electron beam in a direction perpendicular to the direction of deflection of the deflecting electrodes 86a and 86b.
The conventional circularscan system is essential to measure the change with time of a single phenomenon. In general, a light beam incident to the photocathode 82 is focused like a spot, and the photoelectron beam emitted from the spot is deflected to sweep the phosphor screen by the deflecting fields which are formed by applying sine wave voltages which differ in phase by 90.degree. from each other to the two pairs of deflection electrodes.
FIG. 11 is a diagram showing the output of the streak tube as viewed on the phosphor screen 87. As shown in FIG. 11, the sweep images appear circular; that is, the circular scan system is free from the above-described difficulty. Accordingly, the same repetitive light emissions can be observed as repetitive sweeps on each complete circular scan.
When a pulsed light beam's luminance or brightness is measured according to the synchronous scan system which has been described with reference to FIGS. 8 and 9, a number of problems take place because the streak images cannot be added to improve the S/N ratio.
In the case of a specimen generating a fluorescence whose period is longer than half of the period of the sweep voltage employed, the skirt of the fluorescence spreads to the return sweep period, and the streak images formed by the sweeps in the opposite time direction lie on each other. Therefore, the accurate fluorescent period cannot be measured.
Furthermore, if, in measurement of a semiconductor laser beam generated with a period which is just a fraction of one period of the sweep, the laser beam will be generated also in the return sweep period, the streak images will lie on each other on the output surface of the phosphor screen 87. Thus, in this case also, the measurement cannot be made.
As was described above, these problems can be solved by the circularscan system. In order to obtain quantitative data from the streak image, it is necessary to detect the output image with a TV (television) camera. However, processing the video signals of the TV camera can create serious problems.
FIG. 12 shows a streak image obtained using a linear sweep. FIG. 13 is a graphical representation indicating the intensity distribution of the streak image of FIG. 12 on the time axis. In the ordinary linear sweep, the TV camera operates in such a manner that the linear time axis is parallel with or perpendicular to the direction of scan of the image pickup tube. On the other hand, in the circular sweep, the operation is considerably more intricate.
If, as in a time resolved spectrophotometry, a linear sweep is performed with various wavelength rays arranged perpendicular to the direction of sweep, then streak images according to each wavelength as shown in FIG. 14 can be obtained. Therefore the data can be readily obtained by detecting and showing the images with a TV camera. On the other hand, using a circular sweep for various wavelengths, streak images are formed as shown in FIG. 15, and the output image is more difficult to analyze.